1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic process or an electrostatic recording process. In particular, the present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine.
2. Related Background Art
Up to now, an image forming apparatus employing an electrophotographic process as described below has been proposed.
To be specific, a developing rotary is arranged so as to oppose a photosensitive member, the developing rotary including plural two-component developing devices containing toner and carriers. The developing devices of the developing rotary are disposed one by one at a developing portion with the photosensitive member to form a toner image. The toner images formed on the photosensitive member are successively primarily transferred onto an intermediate transfer member. Thereafter, the toner image formed on the intermediate transfer member is secondarily transferred onto a recording material and then heat-melted by a fixing device to be fixed thereonto. The recording material with the fixed image is discharged to the outside of the apparatus.
In such an image forming apparatus, there is a case where a toner image transferred onto a special recording material such as thick sheet or an OHP sheet (light-transmissive resin for an overhead projector) is fixed thereonto. Such a special sheet (recording material) involves a requisite heat quantity per unit area, which is larger than that of plain paper. As a result, the apparatus is structured such that the sheet is applied with a larger quantity of heat than that of the plain paper by reducing a conveying speed (fixing rate) upon passing the sheet through the fixing device.
In this case, regarding an image forming apparatus where a distance between a fixing device and a secondary transfer device is short, when the toner image primarily transferred from the photosensitive member onto the intermediate transfer member is secondarily transferred onto the sheet, a rotating speed of the intermediate transfer member is reduced under control according to a type of recording material, i.e., a speed (fixing rate) at which the recording material passes through the fixing device.
Note that, the rotating speed of the intermediate transfer member is set uniform until the toner image formed on the photosensitive member has been primarily transferred to the intermediate transfer member irrespective of whether the recording material is plain paper or special sheet (e.g., the thick sheet or the OHP sheet).
Note that, as mentioned above, in the case of using the thick sheet or the OHP sheet, the following control is performed on account of the short distance between the fixing device and the secondary transfer device. That is, the rotating speed of the intermediate transfer member is reduced in correspondence with the fixing rate as compared with the plain paper, from the completion of the primary transfer until secondary transfer starts.
After that, the rotating speed of the intermediate transfer member is increased to a normal speed under control for the next image formation.
In the case of changing the rotating speed of the intermediate transfer member as described above, it takes some time for the rotating speed of the intermediate transfer member to stabilize at the changed speed.
Accordingly, before the rotating speed of the intermediate transfer member stabilizes, if the developing device is at a developing position opposite to the photosensitive member, a problem arises in that carriers adhere to the photosensitive member from the developing device.